1. Field of the Invention
The invention generally relates to a calibration circuit and a calibration method, and more particularly, to a calibration circuit and a calibration method which can calibrate an internal resistance without referring to the resistance of an external resistor.
2. Description of Related Art
A high-speed transceiver usually uses an on-die resistor as its termination, and the resistance of the on-die resistor usually needs to be calibrated for the sake of impedance matching. Conventionally, the resistance of an on-die resistor is usually calibrated according to the resistance of an off-chip resistor.
FIG. 1 is a diagram of a conventional calibration circuit 100. Referring to FIG. 1, the calibration circuit 100 calibrates the equivalent resistance of the resistor unit 140 according to the resistance of the off-chip resistor Rext, so as to make the equivalent resistance of the resistor unit 140 equal to the resistance of the off-chip resistor Rext. The calibration circuit 100 has an operational amplifier 110, a low-pass filter 120, and a counter 130. When the calibration circuit 100 starts calibrating the equivalent resistance of the resistor unit 140, the gates of the transistors M1 and M2 are turned on by the gate voltage VG. As shown in FIG. 1, the sources of the transistors M1 and M2 are coupled to a first system voltage VDD, and the off-chip resistor Rext is coupled between the drain of the transistor M1 and the second system voltage VSS. The terminal voltages V1 and V2 of the off-chip resistor Rext and the resistor unit 140 are respectively sent to the positive input terminal and the negative input terminal of the operational amplifier 110. When the terminal voltage V1 is higher than the terminal voltage V2, the operational amplifier 110 outputs a positive voltage, and when the terminal voltage V1 is lower than the terminal voltage V2, the operational amplifier 110 outputs a negative voltage. The low-pass filter 120 low-pass filters the output signal of the operational amplifier 110 to generate a pull-up signal SUP and a pull-down signal SDN. The counter 130 generates a (N+1)-bit control signal S[0:N] according to the pull-up signal SUP and the pull-down signal SDN to control the (N+1) transistors of the resistor unit 140. The (N+1) transistors of the resistor unit 140 are respectively connected with the corresponding resistors R0-RN in series. The equivalent resistance of the resistor unit 140, and accordingly the terminal voltage V2, changes along with the on/off of the N transistors. When the terminal voltage V1 is eventually equal to the terminal voltage V2, the equivalent resistance of the resistor unit 140 is equal to the resistance of the off-chip resistor Rext.
However, a system on chip (SOC) requires an exclusive pin to be coupled to the off-chip resistor Rext. The additional pin and the off-chip resistor will increase the overall cost of the SOC.